1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a an aero gas turbine engine with a bypass fan.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
An aircraft engine is powered by a multiple spool gas turbine engine that includes a bypass fan driven by the low pressure turbine. In a twin spool engine, the bypass fan functions also as the low pressure compressor to supply compressed air to an inlet of a high pressure compressor. The combustor produces a hot gas flow that enters into a high pressure turbine and then a low pressure turbine. The high pressure turbine is rotatably connected to the high pressure turbine to drive it, while the low pressure turbine drives the low pressure compressor which is also the bypass fan.
The fan tip speed is a limiting factor in the design and operation of the engine. Since the fan is directly coupled to the low pressure turbine, the bypass fan operates at the same rotational speed as the low pressure turbine. It is desirable to use as large a bypass fan as possible. However, the size of the bypass fan is dependent upon the size of the turbine blades that drive the fan. To make use of a large turbine fan blade to drive the fan is not desirable because the large diameter turbine would block some of the bypass air flowing through the engine. It is desirable to provide a small low pressure turbine to drive the fan. This would also decrease the size of the overall engine and reduce the weight, both beneficial to improving the overall performance of the turbofan engine.
Another problem with the prior art turbofan engines is that the fan is driven by a turbine that is coupled with the core part of the engine—the high pressure turbine and the low pressure turbine—which is supplied by hot gas flow from the same combustor. Thus, the fan is driven by the combustion gas flow from the combustor that also supplies the core turbines. Since the speed of the low pressure turbine is related to the speed of the high pressure turbine, in this prior art turbofan engine, when the speed of the high pressure turbine increases the speed of the low pressure turbine, and thus the fan, must also increase. The fan speed cannot be separated from the core engine speed.